What Happened in the Stock Market Today

Fitbit shares spiked 7% following a rare piece of encouraging news about the fitness device maker’s holiday sales trends. Its activity tracking software was the second most-popular download among free apps on Apple ‘s iTunes this past week, beating such heavyweights as Snapchat, YouTube, and Instagram.

Seattle Genetics Plunges After Drug Studies Halted on Deaths

Seattle Genetics Inc. plunged the most in five years after U.S. regulators halted several early-stage blood cancer studies following the deaths of four patients who suffered from liver damage. The Food and Drug Administration halted the trials testing Seattle Genetics’s acute myeloid leukemia, or AML, therapy, called SGN-CD33A.